Refractive corrections for human eyes can be characterized into two general categories. The first category is the conventional method of vision correction which corrects for and the eye's focus error and cylindrical error as measured using a manifest refraction. The second category is wavefront-guide vision correction which provides correction for all aberrations in an eye, including focus error, cylindrical error, spherical aberration, coma, and others, measured using an objective wavefront sensor.
The conventional method of vision correction is conceptually limited to a correction of just focus error and cylindrical error. In addition, it is also constrained by the subjective nature of how the manifest refraction determines the eye's refractive errors, particularly the eye's cylindrical error. Cylindrical error is also known as astigmatism, and it causes particular problems because it includes both a cylindrical power and a cylindrical axis.
There are at least five limiting factors associated with a manifest refraction. First, manifest refraction is limited by available lenses in a phoroptor, because a manifest refraction relies on applying corrective lenses and testing vision of the eye subjectively. Focus error is usually limited to a resolution of 0.125 Diopters (D) while the cylindrical error is limited to a resolution of 0.25 D. Second, subjective determination of cylindrical axis can be problematic because a slight variation of cylindrical axis—within only a few degrees—can cause a significant performance difference for a cylindrical correction of more than 2 D. Third, human errors by either the patient or a practitioner—such as an optometrist or optician—cannot be excluded because a manifest refraction involves the subjective responses of a patient to a plurality of refractive corrections, as well as the practitioner's analysis of those subjective responses. Fourth, a manifest refraction is fundamentally a partial empirical refractive solution, because a practitioner conducting the manifest refraction determines an end point for a refractive correction in a time-consuming process. Finally, manifest refraction can also be a time consuming process because it relies on human control of vision optimization with as many as three independent variables which include a focus error, a cylindrical power, and a cylindrical axis.
The drawbacks associated with using a manifest refraction compound with the high tolerance of current lens manufacturing techniques and lead to widespread erroneous vision correction. The inaccuracy of the conventional vision correction method using a manifest refraction leads to a situation where there may be significant differences in a refractive prescription of the same eye by different practitioners, as well as in a coarse resolution of cylindrical power—as large as 0.25 D—universally prescribed for conventional vision correction. Consequently, available ophthalmic lenses in today's ophthalmic industry are also limited to lenses in 0.25 D resolution. Correcting an eye's astigmatism using conventional vision correction is further complicated by the high tolerance in fabricating conventional spectacle lenses. As illustrated in the British standard for tolerances on optical properties of mounted spectacle lenses, BS 2738-1:1998, the tolerance of cylindrical power ranges from ±0.09 D for low power lenses to ±0.37 D for high power lenses. It is safe to say that uncorrected astigmatism by today's ophthalmic lenses is as large as 0.37 D due to the combined errors in the manifest refraction and the tolerances associated with making ophthalmic lenses.
Advanced wavefront sensing that provides reliable measurement of all aberrations in an eye with an objective wavefront sensor is described in U.S. Pat. No. 5,777,719 by Williams and Liang. In theory, wavefront-guide vision correction could provide perfect aberration-free refractive correction for every eye, because all aberrations can be measured objectively. In reality, however, wavefront-guide vision correction also has its challenges. First, manufacturing a lens with precise control of all aberrations across the lens can be complicated and expensive, because it is impossible to use the conventional processes for manufacturing spherical lenses, toric lenses, and aspheric lenses. Second, wavefront corrections require precise wavefront alignment between a lens and an eye at all times. The combination of these issues in lens manufacturing and in wavefront sensing makes it very difficult to achieve wavefront-guided corrections for conventional lenses such as spectacles, contact lenses, and implantable lenses.
Consequently, although many configurations and methods for vision correction are known in the art, all of them suffer from one or more disadvantages. Thus, there is a need to provide improved methods and devices to achieve practical uncompromised vision correction.